Two weeks ago, NASA's Opportunity Rover discovered the largest meteorite yet observed on Mars. Sure, any space debris can be an exciting discovery, but the rock's size has its own cool implications: Mars's atmosphere used to be a lot thicker.

The rock, dubbed "Block Island," is approximately 2 feet long and has a blue tint. The rover took a picture of the formation in passing two weeks back, but the Rover team decided that Opportunity should backtrack and do some more tests on the relatively giant space rock.

The tests revealed that the meteorite is made up of iron and nickle, which makes it very similar to the kinds of rocks that have been found on the surface of the Earth. The big discovery isn't the composition of the meteorite, though. It's the size.

NASA says that any rock of this size would certainly break apart if it were to fall on the surface of Mars today. For such a large meteorite to remain intact, the red planet's atmosphere would have to have been much thicker. The NASA team chalks the existence of Mars's thicker atmosphere up to one of two causes: either Mars has vast stores of carbon dioxide ice that filled the atmosphere in a relatively recent warm period, or the meteorite fell billions of years ago.

Whatever the case, the rock also provides scientists with a way to see how Mars's weather and climate have affected a foreign object, letting them piece together a sort of reverse long-term weather forecast for the planet's surface. And if we ever get that manned flight to Mars we are all hoping for, "Block Island" could become one of Mars's popular roadside attractions, like a Martian Chimney Rock.